Each year approximately 2 million people suffer traumatic brain injuries (TBI) in the U.S. of these about 100,000 die and 90,000 are left with long-term disabilities. Advances in the management of these patients have reduced mortality but done little to ameliorate brain injury. Recent studies in severe TBI patients have suggested that reduced regional cerebral blood flow (rCBF) in the first few hours after injury contributes to secondary brain injury. in order to determine the role of cerebral ischemia in the pathophysiology of TBI, it is important to determine if the reduction in rCBF seen in TBI patients is due to a primary reduction in supply causing ischemia or merely represents a decline in rCBF secondary to reduced metabolic demands of injured brain tissue. Positron emission tomography (PET) is the only technology currently available that can quantify CBF and CMRO2 regionally in humans, and provide indices of the balance between CBF and CMRO2; oxygen extraction fraction OEF, differences in arteriovenous O2 content (A-VDO2), and cerebrovenous oxygen content (CvO2). Barnes-Jewish Hospital at Washington University Medical Center is a Level I trauma center which admits approximately 120 TB1 patients each year. A new Siemens ECAT EXACT HR 47 PET scanner has been installed in the Neurology/Neurosurgery Intensive Care Unit (NNICU) and is currently being used to study TBI patients. The presence of a PET scanner in the NNICU, combined with our extensive experience with the use of PET to detect ischemia, our expertise in the clinical management of TBI patients, and the large available patient patient population gives us a unique opportunity to address these issues. We propose to investigate the prevalence, severity, and duration of cerebral ischemia in severe TBI. Specifically we will measure regional CBF. CMRO2, CvO2, A-VDO2 and OEF using PET in 60 patients with severe head injury (GCS less then or equal to 8) within the first 12 hours after injury and again 24-72 hours latter. These investigations are critical to optimize treatment for these patients. If ischemia is a significant problem in TBI, then management of arterial hypertension and cerebral perfusion pressure will need to modified accordingly. In addition neuroprotective agents currently being studied in ischemic stroke may be useful in TBI as well.